Drop on demand ink jet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an ink jet image is formed by selective placement on a receiver surface of ink drops emitted by a plurality of ink jets, also referred to as drop generators, implemented in a printhead or a printhead assembly. For example, the printhead assembly and the receiver surface are caused to move relative to each other, and drop generators are controlled to emit drops at appropriate times, for example by an appropriate controller. The receiver surface can be a transfer surface or a print medium such as paper. In the case of a transfer surface, the image printed thereon is subsequently transferred to an output print medium such as paper.
FIGS. 4A and 4B illustrate one example of a single ink jet 10 that is suitable for use in an ink jet array print head. The ink jet 10 has a body that defines an ink manifold 12 through which ink is delivered to the ink jet print head. The body also defines an ink drop-forming orifice, or nozzle, 14 together with an ink flow path from ink manifold 12 to nozzle 14. In general, the ink jet print head preferably includes an array of closely spaced nozzles 14 for use in ejecting drops of ink onto an image-receiving medium (not shown), such as a sheet of paper or a transfer drum. Ink jet print heads can have a plurality of manifolds for receiving various colors of ink.
Ink flows from manifold 12 through an inlet port 16, an inlet channel 18, a pressure chamber port 20, and into an ink pressure chamber 22. Ink leaves pressure chamber 22 by way of an outlet port 24 and flows through an outlet channel 28 to nozzle 14, from which ink drops are ejected. Ink pressure chamber 22 is bounded on one side by a flexible diaphragm 30. A piezoelectric transducer 32 is secured to diaphragm 30 by any suitable technique and overlays ink pressure chamber 22. Metal film layers 34, to which an electronic transducer driver 36 can be electrically connected, can be positioned on either side of piezoelectric transducer 32.
Piezoelectric transducer 32 is operated in its bending mode such that when a voltage is applied across metal film layers 34, transducer 32 attempts to change its dimensions. However, because it is secured rigidly to the diaphragm 30, piezoelectric transducer 32 bends, deforming diaphragm 30, thereby displacing ink in ink pressure chamber 22, causing the outward flow of ink through outlet port 24 and outlet channel 28 to nozzle 14. Refill of ink pressure chamber 22 following the ejection of an ink drop is augmented by reverse bending of piezoelectric transducer 32 and the concomitant movement of diaphragm 30, which draws ink from manifold 12 into pressure chamber 22.
To facilitate manufacture of an ink jet array print head, ink jet 10 can be formed of multiple laminated plates or sheets. These sheets are stacked in a superimposed relationship. Referring once again to FIGS. A and B, these sheets or plates include a diaphragm plate 40, which forms diaphragm 30 and a portion of manifold 12; an ink pressure chamber plate 42, which defines ink pressure chamber 22 and a portion of manifold 12; an inlet channel plate 46, which defines inlet channel 18 and outlet port 24; an outlet plate 54, which defines outlet channel 28; and an orifice plate 56, which defines-nozzle 14 of ink jet 10. The piezoelectric-transducer 32 is bonded to diaphragm 30, which is a region of diaphragm plate 40 covering ink pressure chamber 22.
One goal in the design of print heads and, in particular, ink jets incorporated into a print head, is increased printing speed. As is well known, print speed depends primarily on the packing density of the jets in the printhead (jets per unit area) and the jet operating frequency (rate that each jet can eject drops of ink). Individual jet design plays a major role in determining the maximum packing density and the maximum operating frequency. For example, increasing ink jet packing density typically requires decreasing the size of ink jet structures such as piezoelectric transducers, diaphragms, and ink chambers without decreasing the size of drops that they are capable of generating.
In previously known ink jet devices, decreasing the size of the jets to accommodate increased packing density goals may decrease jet efficiency. As used herein, jet efficiency or driver efficiency is defined as the volumetric displacement (drop size) for a given drive voltage. The drop size generated by an ink jet corresponds substantially to the degree of deflection or displacement of the transducer in response to a given drive voltage. The degree of deflection or displacement of a transducer, in turn, corresponds to the magnitude of the drive voltage with the degree of deflection increasing with increasing drive voltage. Thus, decreasing the size of the transducer in known ink jets may require an increase in the deflection of the transducer in order to maintain the same volumetric displacement which correlates to a decrease in jet efficiency for the jet.
Increasing the operating frequency of previously known ink jets may also decrease jet efficiency. For example, in order to increase the operating frequency of an ink jet, ink jet transducers are required that have a natural frequency at or above the desired operating frequency for the jets. The natural frequency of the transducer is related to transducer stiffness. Therefore, higher operating frequencies may require stiffer transducers. Stiffer transducers, in turn, may require increased drive voltages in order to deflect or displace the transducer to a sufficient degree to maintain a given volumetric displacement, or drop size.
As jet efficiency decreases, required drive voltage increases. Increased drive voltage requirements for an ink jet coupled with an increase in total number of jets may result in power supply requirements for the printer to be elevated to unacceptable or impractical levels.